Output impedance of Edcor's transformers? - diyAudio
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Old 24th May 2010, 08:53 PM   #1
Divad89 is offline Divad89  Denmark
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Default Output impedance of Edcor's transformers?

I've completed the Simple SE board a while ago, and is wondering what output transformer is the best for me. At first I decided to use these transformers:

Output transformers: Edcor CXSE25-8-5K
Power transformer: Edcor XPWR059
Power choke: Hammond 193J

But is the CXSE25-8-5K just for 8 ohms speakers or is it also for 4 and 16 ohms? Because at the moment I have 4 ohms speakers.

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Old 24th May 2010, 09:01 PM   #2
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That output transformer is intended for only one specific output impedance. I know Edcor has a few push/pull transformers with multiple secondary taps, but I don't recall ever seeing any of their single ended product with that "feature".

I've been lead to believe that single secondary output transformers have some technical advantages to multi-tapped secondary designs. If you know your speakers are 4 ohm, and you don't plan to shop for new speakers any time soon, why not go for the Edcor CXSE25-4-5K?
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Old 24th May 2010, 09:11 PM   #3
Divad89 is offline Divad89  Denmark
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Will go for CXSE25-4-5K then, thanks...

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Old 24th May 2010, 10:34 PM   #4
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Something else to keep in mind...if you look at the Simple SE "Tubes & Transformers" page, you'll see that many of the designs are happy with a load anywhere between 3K and 5K. Generally, 3K will give a little more power, while 5K will give a little lower distortion.

You might consider getting an output transformer that "splits the difference" if you think you might trade in your 4 ohm speakers for 8 ohm. In other words, consider a transformer with a 6 ohm expected secondary load. The transformer is a ratio device, so you can think of a 6-5K as a 1:833 ratio. If you put a 4 ohm speaker on it, you'll end up with an effective 3300 ohm load. If you put an 8 ohm speaker on it, you'll end up with an effective 6600 ohm load. Either way, you're kinda in the right ballpark. Of course, Edcor doesn't make the CXSE with odd sized secondary ratings.

On last thing to consider - while you can "ratio" an output transformer to a certain extent, some people maintain that they "ratio down" better than they "ratio up". In other words, you need a certain minimum amount of inductance on the primary in order to support the specified (low frequency) bandwidth for a particular primary impedance. If you put a higher than intended load on the secondary, the higher reflected load on the primary will need more inductance to maintain the same low frequency extension. Of course, the other end of the frequency spectrum has its own considerations as well...
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Old 24th May 2010, 10:49 PM   #5
tomchr is offline tomchr  United States
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Another option would be to call up Edcor and ask them to add a 4-ohm output on the 8-ohm transformer. I believe they custom wind every transformer anyway and I can't imagine adding a tap would add that much cost. It's worth a phone call anyway...

~Tom
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Old 25th May 2010, 06:50 AM   #6
Divad89 is offline Divad89  Denmark
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Will try ask edcor about making the extra output tap.
Btw which choke is best for this project? 193J or 193H?

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Old 25th May 2010, 11:28 AM   #7
Divad89 is offline Divad89  Denmark
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But which advantages is there with single secondary output rather than multi-tapped secondary outputs?

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Old 25th May 2010, 01:22 PM   #8
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Quote:
Originally Posted by Divad89 View Post
But which advantages is there with single secondary output rather than multi-tapped secondary outputs?
In my limited understanding, the multi-tapped secondary can have "unused" portions of the winding. It might result in dips or peaks across the bandwidth. If you have a secondary with taps at 4 and 8 and you connect your speaker to the 4 ohm tap, the remaining section from 4 to 8 is not loaded by the speaker. However, it is still coupled to the rest of the transformer and will develop a voltage across it. With no path for current to flow, I'm not sure what effect this will have. I'd guess it also represents some form of stray capacitance that you don't want.

There are some multi-secondary designs (Lundahl comes to mind) where there are four or more secondary windings. These multiple secondaries can be arranged in either a series, parallel, or series-parallel combination to provide various different output impedances. In all cases, there are no unused portions of secondary winding left flapping in the breeze.

Hopefully, someone who better understands this subject will chime in here. I'll be quick to admit that a lot of transformer design is pure magic to me.
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Old 25th May 2010, 01:29 PM   #9
Pano is offline Pano  United States
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It's pure magic to a lot of folks, including those who wind them!

What you're saying about the unused taps makes sense, accept that I can think of a few that have them and are still excellent transformers. So who knows? Maybe it has to do with how they are made.

I wish I had asked for a 16 Ohm tap on my Edcors. Since they are building them to order these days, it would have been wise to ask.
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Old 25th May 2010, 02:29 PM   #10
rknize is offline rknize  United States
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Having unused taps is not the issue, as I understand it. A lot of the magic comes from how the windings are interleaved and in order to get a tap out to the surface, that interleaving gets interrupted. Also, the most common form of secondary winding often uses different gauge wire (largest for the 4 ohm section), so that throws another variable. Plus having to maintain the interleaving "magic" with the extra secondary wire that may never get used anyway...well you get the idea.

To what extent it affects things is hard to say, but not having extra taps obviously makes the job easier for the winder.
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